Strange things afoot at the combo amp motel

[WRC34]

Hi guys,

I haven’t been here in a while. Admittedly, I have come here most often looking for help to diagnose and address unusual issues incurred in my builds. And you guys always deliver. If my numbers ever come in, you’re all getting Ferraris!

But anyway, I’m a busy guy with a job and a family. As much as I hate to admit my grown up responsibilities have slowed down the amp building a bit the past few years. I built my first amp in over two years last spring. It was great fun and I have a video documenting the build on my YouTube channel here:
https://youtu.be/vOXJtgGgJYk?si=yOvpzOyTWu8SNj1g

In the above video everything goes pretty smoothly and the end result was something I was happy with. The amp never saw long, sustained use though. And now recently at a band rehearsal where I brought the amp for our guitarist to use (I was eager to show off my work to him and for the amp to get some ‘burn in’ time) after a couple songs I noticed this ugly blarting, crackling distortion. He had some kind of fancy pedal strip which had chorus, reverb, high gain, overdrive, etc. I thought the signal was just too much and blew the speaker. Upon getting home the next day I turned the amp on and sure enough palm muting at loud volume produced a crackling sound which seemed to come from the center of the speaker and was similar to the sound of other blown speakers I’ve had over the years. Shit. To be sure I disconnected the speaker in the cabinet and ran the amp as a head through a separate 2x12 on the other side of my workshop. The amp sounded great, no crackle, no crispiness. Then, back to the speaker inside the combo and the harsh sound was back. So, I concluded, the speaker must be blown. Dammit.

A couple weeks went by until I had another chance to get to it and in that time I developed an idea to install the speaker in a separate cab to determine if indeed the speaker itself was blown or if the problem was more elusive. Well, wouldn’t you know, the speaker I suspected being blown when installed in a can separate from the amp was in fact…fine. High volume, Palm muting. Power chords. Fast riffs. No issues. WTF!

To be clear, the crackling, problematic sound is coming from the speaker itself. It’s not a loose screw or piece inside the amp getting rattled loose from the vibration in the cab. I can only think at this point there is something electronically inside the chassis which is reacting to the vibration of the speaker when installed in the amp. Is there a list of usual suspects in a case like this? Like, what?

Any and all input is greatly appreciated. I will try to generate a couple short clips showing the amp run through a separate speaker cab vs the speaker re-installed in the amp.

Here are some pics for fun and maybe reference.

Hope you guys are all doing well!

[WRC34]

Here’s the layout drawing

[timrobertson100]

Frustrating… I once had an intermittent fault that sounded similar to what you describe that turned out to be a dodgy cap on the tone stack.

[WRC34]

Anything helps, thanks Tim!

[Stevem]

This is when using process of elimination works 95% of the time .

I would start by removing one output tube at a time to see if the cracking follows one tube or the other.

Do you have a spare rectifier tube?

[Snicksound]

Over my near 20 years of building amps, it always came down to the following two reasons when something was afoot:

1- a cold or bad solder joint (can work intermittently, heat makes metal move and things change)
2- a bad tube

One time a cap lead had broken (amp worked but would sometimes go into odd motor boating). Another time I learned not to use Silver Mica caps in the tone stack.

[Bobwith2Bs]

In my experience, something like this is a prime candidate for some chopstick testing to replicate the issue.

If the speaker works in a different cab, and the amp works with different speakers, then the most likely cause is that the vibration of the speaker while in the combo cab is causing something in the chassis to vibrate/move in a way it doesn't like.

This could be a tube, a bad solder joint, or even just a bad/dirty tube socket.

I would start by letting the amp be powered up on it's own and wiggling the tubes and tapping with a pencil or a pen to check for microphonics, and if that doesn't produce anything would move towards poking at boards/components/solder joints with a (NON CONDUCTIVE) chopstick to try to provoke the issue.

[ampdan]

I agree with Bob. I experienced the same symptoms before. It turned out to be the power tube cathode resistor was acting as a tuning fork.

[WRC34]

Hi guys!

As noted I don’t get as many opportunities as I would like for loud testing, etc.

I understand and generally agree with the basic principle of most issues being a cold solder joint or bad tube. However, I have run into some weird ones over the years. One amp that really gave me a hard time had a similar setup to this one - cathode biased with separate bias resistors for each output tube. Nothing helped until I swapped them out for a shared bias resistor. After that, totally smooth good sounding amplifier. But why? We never quite got down to the bottom of it but, ampdan,
I do find your point of the cathode resistor acting as a tuning fork to be interesting. Although in the case of this amp would that be the reason for the clacking, unpleasant sounding distortion coming through when pushed?

I also had an amp I built for a client/friend during the pandemic that I used shitty preamp sockets in. Sure enough, the sockets caused all kinds of instability issues and when replaced with higher quality ones the issues disappeared. I have a video about that one on ye olde YouTube channel as well

Being fortunate enough to have a modest stash of tubes I have replaced the output tubes, then the rectifier, then each preamp tube and the amp remained pretty stable.
But I also haven’t had time to remount a speaker in the cab so these tests are not wholly conclusive. I have now run it through multiple extension cabs and they all produce good, stable sound.

I was able to choppity chopstick a bit in there with the volume turned up and the only weak point was the coupling cap coming off V1 connected to the volume/tone setup. And even then it wasn’t consistently noisy.

Something to note is prior to the new Weber 12A150 I had a trusty old 50 watt Weber Alnico Blue Dog in the amp which I have beat the absolute snot out of since 2009 and it sounded fine. I wanted a bit livelier sound so I went for the 12A150 (which I had sitting around but had to get reconed) I have blown many a Celestion over the years but never a Weber until this 12A150, and even then I don’t think it blew as much as it developed an issue over time.
I have this funky pair of Alnico magnet Carbonneau hi-fi speakers that sound kind of interesting and have blasted the amp through them without issue. I also had this weird pair of Japanese made Alnico magnet Pioneer 12” speakers a while back, also hi-fi speakers not made for guitar but they sounded absolutely fantastic. I put the cab in storage for a while and chipmunks friggin chewed through the paper cones. Man, that was a bummer discovery!

Ok well… I’m trailing off here, but will report back once I get a speaker mounted back in there and have a chance to thrash it a bit. Debating on putting the Blue Dog back in, might be a good way to narrow things down.

[TUBEDUDE]

As an aside, it's hard to tell in 2D, but your OT leads aren't touching the final cathode resistor are they?

[WRC34]

Tubedude - you mean the zip tied leads that aren’t in use?

[Phil_S]

Lots of good suggestions already made. I'm not saying any one of them is right or wrong. You may have more than one problem. I have one more suggestion.

There is no sense testing with speakers in another cab. You've established that the condition only exists with that speaker in that cab with that chassis. Mount the speaker before you do anything else. I"m going to assume the problem will still be there when you mount the speaker. As noted, things can be hard to see in a photo. There is one power tube that looks close to the speaker magnet. Maybe there is some weird coupling between the tube and the magnet? Or, if not a tube, them maybe some component location in the chassis relative to the magnet (I think not likely)?

Though not a permanent fix, maybe you can drill a few more screw holes in the baffle and rotate the speaker away from that tube. You only need 4 screws in the speaker for a test. If that solves the problem, a new baffle will be needed that is properly centered on the speaker. Or you might try to front load the speaker, making some horizontal space by moving the speaker forward.

[TUBEDUDE]

Tubedude - you mean the zip tied leads that aren’t in use?

No, the green and black OT leads attached to the output jack.

[LOUDthud]

I'm not seeing a ground connection to the pots in your layout.

[WRC34]

Hi guys,

Thanks for all your help, questions and suggestions!

LOUDthud - good eye catching the missing wire in the drawing. IRL there is indeed a wire grounding the volume / tone setup. See attached photo where red arrow indicates ground wire and blue arrow shows grounding point.

TUBEDUDE - the green and black secondary OT wires don’t come into contact with the cathode bias resistor or bypass cap anywhere


Phil - I did re-mount the speaker back in the cab. The issue persists. Front loading I think isn’t possible with the way the grill cloth is attached to the baffle.

Man I really thought I had it dialed this past week but alas, I didn’t. The suspect was coupling cap coming off V1 connected to the volume pot. Replaced and still crackling. Photo shows new cap - a silver PIO with black wire soldered to it and yellow heat shrink. I made a short video explaining where I am with it all. It should be up on my YouTube channel within the next day or two. I will let you all know when I post it!

I have a few ideas - maybe I need to switch to a shared cathode bias resistor like I have on almost every cathode biased amp I’ve built . I also like to use carbon comp plate resistors in my builds (I know, I know) I use only 1 watt rated or higher though to thwart (or further prolong) the inevitable noise and failure. Perhaps one of them is compromised and reacting poorly to the vibrations in the cab. Or maybe I need to change the values of some components to “tame down” the circuit. The thing screams. Maybe it’s too wild? Either way I’m about to leave for a 5 week trip and I won’t have another opportunity to work on it til late March :/

Thx again everyone